MINNEAPOLIS, Minnesota (CNN) -- Security is being beefed up at the site of last week's bridge collapse after 16 people were arrested for trespassing and hindering the investigation, Minneapolis police said Wednesday.

Authorities are adding cameras, motion detectors and other technology that will alert police to intruders, Capt. Mike Martin told reporters during a news conference.

"The most important part is: Maintain the honor and the dignity [of the scene]," Martin said.

He said the state Department of Transportation is assuming control of security in the area, which is considered a death-scene investigation site.

Five people are confirmed dead in the collapse of the Interstate 35W bridge August 1.

Eight are missing and presumed dead. Their bodies are believed to be trapped under chunks of concrete and other debris and in crushed vehicles.

One car was taken from the water Tuesday because recovery divers said it was obstructing their search for victims.

Navy and FBI divers -- who are working from 4 a.m. to 10 p.m. to recover evidence and retrieve the remains -- identified four more empty vehicles to be removed to allow better access to the wreckage, the National Transportation Safety Board said. All four were removed by late Wednesday, the Hennepin County Sheriff's Office said.

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Navy Secretary Donald Winter, who visited the collapse site Wednesday and met with some of the divers, called it an "amazing tragedy." He warned it would take a while longer to finish surveying the damage and complete the recovery.

"Right now we're going through a survey process to make sure we understand what is down there," Winter said. "We'll be here as long as we are needed and as long as we can really help."

The agency also reported it has interviewed members of the crew of a dinner cruise ship that was in a nearby lock at the time of the collapse.

Martin said the recovery process will not be easy.

"As hard as it is for us to talk to people about this and to explain it in a way that is not too gruesome, those vehicles have been crushed, for the most part. There may be areas where they are protected that are inside that we haven't seen," he said.

"But this is not going to be a case where they cut away rebar, maybe cut into some of the superstructure, and then reach into a window to recover the body. This is going to be a process of having to, most likely, pull these vehicles out and do a long-term extrication -- taking apart the vehicle to recover evidence."

The police chief encouraged anyone who witnessed the collapse and hasn't talked to authorities to call a toll-free hot line set up by the National Transportation Safety Board: 1-866-328-6347.

He particularly asked for anyone who could talk about reports of someone in a kayak near the bridge to come forward.

The police also released a telephone number Wednesday that relatives or victims can call to retrieve personal belongings such as purses, clothing and laptops that were left in vehicles.

"We want them to leave a message with information on the car that they left," and any possessions that might be inside, Deputy Chief Valerie Wurster said. The number is 612-673-3676.

She said police have retrieved few items so far, but "we're hoping we can release those effects very, very quickly" once each vehicle is removed from the water.

Michael Asleson, spokesman for the Minnesota Highway Patrol, said the process of reconstructing the collapse continues. Investigators have mapped the location of 88 vehicles, trailers and construction equipment, he said.

State engineers recommended in 2000 that the bridge be replaced or redecked, the Minneapolis Star Tribune reported Wednesday.

That 2000 recommendation followed reports beginning four years earlier that raised concerns about the bridge, Star Tribune reporter Pat Doyle said Wednesday on CNN. Engineers were so concerned, "They ended some reports with exclamation points," Doyle said.

The bridge was undergoing minor resurfacing when it collapsed and was not scheduled to be replaced until 2015 at the earliest, Doyle said.

The bridge collapse sparked concern about highway spans across the country, and U.S. Rep. James Oberstar, D-Minnesota, pledged Wednesday to introduce legislation aimed at repairing structurally deficient bridges nationwide.

Oberstar, who is chairman of the House Committee on Transportation and Infrastructure, pointed out to reporters that nearly 74,000 bridges are classified as "structurally deficient" by the U.S. Department of Transportation.

"If we don't want another I-35 collapse, we have to address that portfolio," Oberstar said.

Minnesota state bridge inspectors had classified the bridge as "structurally deficient" for at least 17 years.

Oberstar's draft legislation, which he said he will introduce when the House reconvenes September 5, would improve bridge inspection requirements, provide dedicated funding for improvements, distribute funds based on public safety and need, prohibit earmarks by Congress and the administration and establish a trust fund for bridge repairs and replacements.

The effort, Oberstar said, would be funded by a 5-cent increase in the gas tax, which he estimated would raise $8.5 billion yearly. The program would end after three years.

"We cannot wait for another tragedy," he said. "We must, and we will, act decisively."

Public mourning for the victims of the Minneapolis bridge disaster continued Wednesday. Gov. Tim Pawlenty ordered flags be flown at half-staff at the state Capitol from sunrise to sunset.